The present invention is directed to a method of and an apparatus for removing residual plastic from plastic processing equipment. Such equipment can include molds, extruding screws, extruding pipes and the like. The present invention can also be used for removing the residue of materials other than plastics, such as enamel and varnish.
The equipment used in processing plastics has traditionally been cleaned with either heat or chemical solvents. However, heat can degrade the equipment; for example, it can degrade steel tools by altering the grain-structure of the steel. Chemical solvents may also degrade certain tools; more importantly, it is often difficult to dispose of the used solvents in an environmentally acceptable manner.
Some industries use impact cleaning methods such as scraping and blasting. For example, in the printing industry, cans having ink residue are frozen, and the frozen ink is scraped off. However, such methods are not suitable in all industries. The scraping or other impact may damage high-precision equipment; also, some tools, such as extruding screws, are too intricate to scrape efficiently.
It is also known in the art to clean equipment with pressurized gas, e.g., CO2. However, the force of pressurized CO2 does not suffice to clean the equipment in all cases.
The combination of extreme cold and blasting has been used to deflash molded articles, or in other words, to remove the residual material left on the articles between the interfacing mold surfaces. An example of this technique is taught in U.S. Pat. No. 4,979,338 to Schmitz, II et al. Similar techniques have been employed to remove an adherent coating from an article, as taught in U.S. Pat. No. 4,627,197 to Klee et al and in U.S. Pat. No. 5,761,912 to Popp et al. However, those techniques do not effectively and efficiently remove all of the residue without the need for impact cleaning.
In view of the foregoing, it should be apparent that there still exists a need in the art for a method and apparatus for removing plastic residue from plastic working tools and equipment so as to harm neither the equipment nor the environment. It is, therefore, a primary object of the invention to remove residual plastic from plastic processing equipment through a cold process so as not to degrade the equipment.
It is another object of the invention to remove residual plastic from plastic processing equipment in an environmentally acceptable manner.
It is still another object of the invention to remove residual plastic from plastic processing equipment without the use of scraping or other impact.
To achieve these and other objects, the present invention is directed to an apparatus and method in which the equipment is agitated while the temperature is cycled through a variety of heating cycles, some involving extremely cold temperatures. Since different materials have different degrees of thermal expansion and contraction, the combination of the thermal cycling and the vibrational energy breaks the adhesive bond between the plastic residue and the material of which the equipment is made, typically steel.
The contaminated equipment is loaded into a fixture, which is placed in a thermal chamber. The chamber is heated, typically to 250-300xc2x0 F. and held at that temperature. The high temperature both removes excess moisture from the chamber and thermally expands the equipment, so that the plastic xe2x80x9cbreathesxe2x80x9d and starts to break. Then, the bottom of the chamber is flooded with liquid nitrogen (LN2) so that the chamber rapidly cools to xe2x88x92315xc2x0 F. The vapors of the LN2 cool the equipment and the plastic and the equipment shrinks more rapidly than the plastic. During the cooling, the fixture is agitated to vibrate the equipment, thus assisting in the separation of the plastic from the metal. Impact is not required to remove the plastic.
The chamber is then heated to xe2x88x9210xc2x0 F. to achieve a phase change in the plastic, namely, from ductile to brittle. The temperature in the chamber is then cycled twice between xe2x88x9250xc2x0 F. and xe2x88x9210xc2x0 F. to induce a phase change in the plastic and thereby to fatigue the plastic. Then the temperature is elevated to xe2x88x92150xc2x0 F., held at that temperature for a time and plunged back down to xe2x88x92200xc2x0 F. Throughout the various temperature cycles, the fixture is agitated. The repeated phase changes fatigue the plastic. That fatigue and the differing degrees of expansion and contraction of the steel and the plastic allow a complete separation between the plastic and the steel under the agitating force and in particular prevent the plastic and steel from bonding back together.
At the end of the temperature cycling, the chamber is brought up to xe2x88x92100xc2x0 F. and then to ambient temperature. When the nitrogen gas is vented and the chamber is opened, the fixture can be removed to remove the equipment therefrom. The plastic separated from the equipment is at the bottom of the fixture and can easily be removed.
The chamber is preferably made of stainless steel on the inside. Of course, it is preferred that no component exposed to the thermal cycling within the chamber be made of plastic or any other material to be separated from the equipment, so that the chamber does not destroy itself. The fixture is mounted on rails, and an agitating motor is supplied outside the chamber, with a rod or the like extending into the chamber to agitate the fixture. The rod extends through bearings capable of resisting the operation of the chamber. Heaters, fans, and an inlet for the introduction of LN2 into the chamber are provided. The nitrogen vapor can be vented in various ways in accordance with the environment in which the chamber is to be used.
The above operations can be performed under the control of a computer. Sensors and control devices can be provided to effect such control.